During the combustion of fossil fuels, sulphur in the fuel is released in form of sulphur oxides (SOX). Other pollutants are primary particulate matter and nitrogen oxides (NOX). It is well known that air pollution seriously impact peoples health and the environment. It is also well known that sulphur dioxide and nitrogen oxides are the major precursors of acid rain.
So far, legislation and setting environmental standards of emission control for international shipping has lagged behind the land-based emission regulations. Present regulations include caps on the sulphur content of fuel oil as a measure to control SOX emissions. Special fuel-quality provisions exist for SOX in emission control areas and there are substantial reductions in the allowed fuel sulphur limits to be expected in the near future. The MARPOL Annex VI legislation, which went into effect in May 2005 following specifications from several European Union directives, has curbed the impact of marine diesel on the environment. By 2015, the legislations will be even stricter regarding by way of example fuel sulphur limits and NOx cap.
There are different emission reduction possibilities, either taken alone or in combination. One possibility is to use new fuels such as distillate fuels or low sulphur fuels. Another possibility is to further develop methods to control the emission of SOX such as wet scrubber technologies using alkaline agents such as NaOH-solutions, or dry scrubber technologies using granulated limestone (Ca(OH)2).
It is well known in the marine industry today for the purpose of reducing SOX and NOX gases in the exhaust from a ship to apply Exhaust Gas Cleaning (EGC) for SOX reduction and Exhaust Gas Recirculation (EGR) for SOX and NOX reduction. Both the EGC and EGR processes may apply a wet scrubber technology using an alkaline agent such as a NaOH-solution for removal of SOX. However, since the EGC process is applied on the low pressure side of the exhaust source and the EGR process is applied on the high pressure side it is not possible for them to share the same wet scrubber.
Further, there are different requirements regarding the need for removal of particulate matter, such as soot, oil or heavy metals. The EGR process requires full flow cleaning of all liquid before introduced into the wet scrubber to thereby minimize the amount of particulate matter otherwise entering the engine, causing deposits therein. The EGC process on the other hand is not particularly sensitive to particulate matter in the scrubbing process, but soot precipitations on deck may occur if the amount of particulate matter in the water used in the scrubbing process becomes too high. Thus, it is recommended that each process is provided with a separator, such as a centrifugal separator.
When capturing SOX in both processes, the content of dissolved sulphite and sulphate increase, causing the density of the scrubber water to increase. There is a maximum amount of allowed dissolved salt in the water since precipitation of solid salt might occur. Also, the density of the water might become too high for the centrifugal separators to work optimal.
Accordingly, even though both EGC and EGR processes use a wet scrubber technology, they have partially different requirements, whereby they cannot easily be combined.